There's a lot of good suggestions here, and I'll try to add to them.
What I normally say is that I spend my time doing other things, as mentioned by someone else. I usually mention what it is right away, or, if I feel they might ask, let them ask. I also usually mention the most interesting and "off the wall" thing I've done.
Them: "You don't know who {XYZ sports star} is? What's the matter with you?!"
Me: "I spend my time doing other things, like building my 4 ft by 8 ft capacity CNC mill." *
Them: "What the {bleep} is a CNC mill?" - or - "You did what?!?!"
Topic has changed from my perceived ignorance and I show them that even though I lack some information, I have a whole storehouse of information they've possibly never even considered. If they are really rude and refuse to acknowledge that what I do is even slightly interesting, I make sure not to be around them anymore.
Life is too short to spend time on people who don't like you and you don't like.
Depending on how rude they are being when they make that first statement, I'll use an alternate to "other", such as: better, more interesting, more useful, or a variety of other terms that make their topic seem as insignificant as they currently see me. If I know the person and realize it's just an outburst, I'll leave it as "other", but if it's some obviously moronic idiot that sneers at anyone who isn't him, I'll lay it on real thick.
As always, just make sure that you are telling the truth and not backing yourself into a corner by talking about something you don't know anything about or can easily be proven you didn't do. You might also want to have proof of your claim (pics, website, etc.), so that when/if they ask for it, they can see it for themselves.
Not-Me: "I'm the first person to visit Mars and Saturn, and I think your sportsball is almost good enough to be played by drunken, below average IQ siminans. Oh, right, that's you!"
As amusing as that might be, the first part is obviously not true and will reduce your credibility to less than zero, which means that when you get beat up for the rest, no one will really care.
Caution
Don't overdo it. This may be perceived as you stealing the show. Even if it's something spectacular, like winning a Nobel Peace Prize, play it down a little and try to blend back into the normal conversation. If people are truly interested, they will prompt you to keep the conversation going as is, rather than you pushing it on them, even if all you're trying to do is drown out that first person.
After your first few comments about your achievement(s), you can try steering the conversation back where it ended, right before that initial outburst derailed everything. After a 1-2 minutes (or maybe just 30 seconds) of "defending your honor", you can (or should) let it go and get back to what everyone else was doing.
At this point, you've either corrected the initial insult and should let it go, or you'll be insulted again and know to steer clear of that person in the future.
I don't always mind getting into arguments, so take my suggestion with a grain of salt. Sometimes it works and sometimes it causes more problems. Regardless, "Keep calm and carry on."
* Yes, I really did build a 4'x8' capacity CNC mill. http://www.EricsGear.com