I just found this out, so I'm posting it, since it might be useful both as a reminder, and for others to read.
Suppose you have a "classical" Subversion web interface, like svn.openstreetmap.org. This will show you the last revision by default.
What if you want to see a particular revision? I found this by googling a bit:
http://<yoursvn>/!svn/bc/<revision>/<path>
So, say, using the above example, you want to see the sites/ directory
of revision 20000: here you are (compare with current).