In addition to rsync, one useful tool is the unix diff command. As mentioned in other hints, diff can not only compare two files, it can, by using the -r option, walk entire directory trees. Intelligent Comparison Compare files and folders using simple, powerful commands that focus on the differences you're interested in and ignore those you're not.

1. Mac Folder Comparison Tool
2. Mac App Compare Directories Free
3. Directory Compare Free

I use rsync alot myself to copy files from and to work. Any projects Im currently working on for an extended period of time get their own folder and rsynced between work and home.
Though Im usually good about rsyncing before I leave work or home, I forget occassionally.
So I dont accidentally overwrite any changes - I usually use the -n flag. That does a 'dry run' of rsync showing possible changes without actually making them.

Some time ago I was looking for a tool to binary-compare two directories too. Diff is a bit of an overkill in this case since I was not interested in the details of differences between the two dir structures, I just wanted to know whether they're the same or not.

I've found only one GUI application that would satisfy my needs, it was Kdiff3. However there's a lot more simple and elegant solution. :-) Start a shell and enter the dir that you want to compare with another, then run a 'find':

Mac Folder Comparison Tool

cd /path/to/dir1
find . -type f -not -exec cmp {} /path/to/dir2/{} ';' -print

Mac App Compare Directories Free

This will print the name of each file that differs between the two directories.

It seems to me that any items uniquely found in dir2 won't be reported by the find command.

make sure to format this command exactly the same and including these: ' ' when needed
find . -type f -not -exec cmp {} /path/to/dir2/{} ';' -print
I really just wanted to thank you and everyone in this thread. Relocating and cleaning up my mothers age old backup folder that is filled with duplicates has been made so much easier.
You have all my grattitudes.

Some people may haven't found the feature, yet, but Toast has a 'Compare' function hidden in the Utilities menu, it'll provide a nice colored output of the differences.
And TextWrangler also supports diffing of folders.

I prefer the use of Subversion to stay in sync with several computers with different OS-es.
There good clients available for OSX like svnX

I too am trying to keep a desktop and a laptop in sync - both running OSX. Of course the laptop has much less hd space which means I need to be selective. I have been using Unison very successfully so far for documents in general and for certain application preferences. It is very fast and a pleasure to use.
The complications come in due to the fact that applications do not always play nice with copying preferences and the like. For some apps, I haven't figured out which pref and support files are safe to copy. For example, taking a simple-minded approach to syncing causes MS Office to sort of re-install itself each time. Firefox preference copying is also not obvious to me.
Is anyone else sync'ing app preferences and executables?
Also, I am curious how people are using Subversion.

This hint makes me realise I'm not alone in thinking there must be simpler solution to the problem of keeping a directory in sync between work and home computers, without having to lug an external hard drive around. I need to keep a large (about 5GB) directory of pdf files in sync between a work PC and a home Mac, all sorted into appropriate sub-directories. I currently do this manually by copying any changed files onto a flash memory stick and then copying them onto the other machine when I get home. The flash drive is not big enough to contain a copy of the entire directory, but can easily hold just the changed files. Ideally, some little program or script would identify added or changed files, copy them to the flash drive, then put them into the corresponding sub-directory when the flash drive was connected to the other machine. Then it would repeat the process in the opposite direction.
Synchronization utilities are no good as they require a disk big enough to hold the entire directory. Unison is no good as it requires a network connection between the 2 machines (which most admins baulk at).
Anyone have any other ideas?

I too am trying to keep a desktop and a laptop in sync - both running OSX. Of course the laptop has much less hd space which means I need to be selective. I have been using Unison very successfully so far for documents in general and for certain application preferences. It is very fast and a pleasure to use.
The complications come in due to the fact that applications do not always play nice with copying preferences and the like. For some apps, I haven't figured out which pref and support files are safe to copy. For example, taking a simple-minded approach to syncing causes MS Office to sort of re-install itself each time. Firefox preference copying is also not obvious to me.
Is anyone else sync'ing app preferences and executables?
Also, I am curious how people are using Subversion.

I have a question about this hint.... I'm not too well versed in terminal, but I'd love to do this with my two music folders (one on my external HD,and one on my internal). Do I put all of that code into terminal (including the '|'s) in one line, or do I run them as three separate commands in succession?
Also, where does this diffs.txt show up?
Thanks!

Put it all on one line. The diff.txt file will show up in your working directory. If you just started terminal and did not change directories, this will be your home directory (i.e., not the Desktop or Document directories).
One tip: to quickly enter the two folders you want compare, first type the diff -rq command. Then drag-and-drop the first folder onto the terminal. The path will automatically appear. Then drag-and-drop the second folder. Then type the rest of the command.

Directory Compare Free

Users trying to sync home directories between two computers (say a laptop and a desktop) may find Apple's Portable Home Directory feature of Mac OS X Server helpful. I haven't used it, so I can't say how well it works.