These days, the helmet I wear most often on the road is a Bell Phi Pro. They were about $75 when new in 2002, and an excellent buy back then. You can still find some around for $45 or so. The Bell Alchera is very similar$85, but often on sale for $55 or less at places such as Performance (www.performancebike.com). The Phi fits me well, has good ventilation, and I think is a real bargain.
For mountain biking, I've been wearing a Giro Havoc ($70; www.giro.com), a pretty decent mid-priced ATB lid. It has a visor, which is something you find more often in mountain bikinghelps keep the sun off your face and mud out of your eyes, plus provides a little face protection from branches and the like. But really, there's nothing special about a visor. I sometimes road bike with a visored helmet, and sometimes mountain bike with an un-visored helmet. Generally, mountain-bike helmets have larger holes on top than road-bike helmets, the logic being that you're going slower on a mountain bike so you have to rely a little more on convective cooling (evaporation) than flow-through cooling.
Otherwise, pay more and get more ventilation. Giro's E2, for instance, is a wonderful MTB helmetextremely light, and with 24 vents that make it plenty airy. But, it's $125. Similarly, the Limar 909 road helmet gets plenty of air across your headit literally has a mouth-like opening in front. Mind you, it's in the high price range, too, at around $150.
One tip: Wear a sweatband. That goes a long way toward keeping sweat off your eyes. It also makes you feel cooler, as the drying sweatband is much cooler than the ambient temperature.
Get the scoop on essential road- and mountain-biking gear in Outside's 2004 Buyer's Guide.