I've been using Easy Thumbnails for a couple of years now, really great piece of software. Even compresses pictures where you don't resize or drop quality! Irfanview looks nice too, though, and I may give it a try.
Something that's not a resizer, but vastly improves the quality of some photos, is a little program I found called DCEnhancer, which is a freeware version of DC AutoEnhance, both to be found at MediaChance. (Direct link to the products here.) I find it absolutely invaluable when I'm enhancing pictures taken on my digital camera - I go to a lot of gigs, and usually my photos from them are pretty dark and dull; this totally brings them to life (although it can whack the file size up a bit, and that's where Easy Thumbnails comes in handy ). On one of my pictures, when I enhanced it I discovered there was a band member in the resulting picture who you couldn't even see in the original one! AutoEnhance has a batch conversion facility in it (though personally I like to check my pics one by one) and a lot more features than DCEnhance, but it's $35 (you can get a 30-day trial on the site). DCEnhance is completely free though... Have a look and see what you think