Type in yourcompetitor.com at google. When the page comes up that has just the link to your competitor click sites that link to yourcompetitor.com. That should give you an idea of what sites are linked to them. If you are looking for stats your out of luck.
Things like wordtracker, overtures search for keywords tool, and other software/information can at least give you an educated guess.
Also, some servers even openly publish their statistics (especially .edu sites). This may not be relevant to a .com but it can give you good information about general trends across websites and how other people get a hold of traffic.
Check out Brett's Viral Marketing [webmasterworld.com] thread and a little deduction can go a long way "i guess" ;)
Check out what your competitor is doing as mentioned above IMO...(google aint bad at finding all those links (alongside something like ATW).
If your competitors use a web-based traffic logging systems, you might get a peek at their logs. There was a thread a while back about one particular service that had open logs, maybe someone recalls the name of the service...
And then there are data mining companies like compete.com, where you may sign up for $$ and keep tabs on your competitors. A lot of people, though, will ban data mining data bots, so I don't know how "real" the stats they'd deliver for such a competitor may be.
Reminds me of SPECTRE and James Bond - sitting at command central and spying on every move the opposition makes. I s'pose one could have worse hobbies.
Good free options - previous - is probably see if they publish their log files or do a little Google-checking.
If you download the GoogleToolbar (IE only) it has a backwards links functionality. You go to you competitors site, click backward links and a Google SERP page will appear with all the sites with PR3 or above who link to him.