It's been a fringe litterature thing for a long time, before the term 'steampunk' was even coined. Never became as big as cyberpunk, but it stayed there, that "what if our ancestors had had science more advanced than they actually had? What would have happened and what would it have looked like?".
In a way, Jules Verne and HG Wells are the precursors of Steampunk, really.
The first example that comes to mind is indeed Space 1899. Most steampunk tends to take place in the final years of the 19th century - age of the steam engine, obviously. But it's not too hard to expand that a bit. After all, the Greeks and Romans knew about steam and almost developed primitive steam technology - for some reason they failed to see the interest apparently.
As for why it's become so popular lately... I honestly can't tell. I didn't even know it had become popular - I thought I'd have to spend the entire week-end explaining to people what my costume was and what steampunk meant, but mostly people recognised it right away.
In the MMORPG City of Heroes, one of the main bad guys group, Nemesis, is steampunk-themed, maybe that contributed. I know back in France, in the past 3 or 4 years 2 comic series came out with a serious steampunk aspect to them (check out Hauteville House:
http://www.editions-delcourt.fr/catalogue/bd/hauteville_house_1_zelda and Empire:
http://www.editions-delcourt.fr/catalogue/bd/empire_1_le_general_fantome) but these weren't translated that I know of.
These two were a big inspiration for me to try and make this costume, combined with Naomi Novik's "Temeraire" series. No steampunk, but the Napoleonic wars with an Air Corps of dragons on all sides. Really cool, and I figured: "Hey, if someone made Napoleon with dragons, why not in space?".
EDIT: actually, I'm thinking a good bet on why steampunk became popular suddenly is it's inclusion in World of Warcraft (I think gnomes have steam-powered machines, right?).
It's been around in Warhammer for a long time, but WoW has such a wide customer base with such a number of 'flavour-of-the-month' followers I wouldn't be surprised if it was the impetus for this sudden revival (no offense to WoW players, I'm not saying you're ALL like that; simply that the game seems to attract a lot of that kind).