You may recognise him from my avatar. He's my sigfig.
The concept of a "sigfig" seems to be commonplace on a lot of LEGO sites I've visited. Not so much here on Brickipedia; a quick search tells me there are no results matching my query, and with one or two possible exceptions, the regulars here don't seem to have one. It's not the only site where sigfigs aren't a big deal, but they're an idea I came across pretty much immediately after ending my Dark Age and took to quite happily, so I'm here to spread the love.
Basically, your "signature figure" is a particular minifigure that you've adopted for some reason, and use as an avatar in anything that might need one. Not just the avatar image beside your screen title, but in your LEGO models and anything else that could use your personal touch. The Eurobricks forum is particularly fond of running contests featuring users' sigfigs. I've even used Dunjohnbot as a literal signature on some creations.
Some people take a full, ready-made minifigure and adopt it without making any changes. Most people build their own. A few customize the parts and printing to make something completely unique, and in one or two rare cases like mine, the sigfig isn't even a fig at all, being built out of other parts. It doesn't really matter what form your sigfig takes. What matters is that the sigfig means something to you.
I didn't make my guy out of bricks just to be special or awkward (and believe me, he can be awkward). I've had him for much longer than I've been an AFOL. A good few years ago, I saw some tiny LEGO Star Wars sets in a local supermarket. I needed something cool to sit on top of my computer screen at work, so I got two (the same models in 4484 Mini X-Wing Fighter & TIE Advanced, but I'm positive they were packed separately). They were cheap and interesting, and would probably make good conversation pieces. When I switched jobs a little while later they were relegated to my bedroom windowsill to get dusty and yellow. One evening a few months later, I started messing with them, and one of the end results was this wee robot. I thought he was such a cute little thing that I didn't break him up and toss him with the other parts into the junk drawer. Instead, I blu-tacked him to the top of the television. When I eventually did end my Dark Age I replaced his discoloured parts, coordinated his scheme a bit (his legs were originally sand-blue) and knighted him my official sigfig.
And named him. Most people's sigfigs have the same name as their owners but I'm not really a robot....
So that's where my dude comes from. If anybody here has a sigfig, I'd love to hear why they chose it. And for the people who don't, would you not be tempted to promote one of your little plastic worshippers?