A treasure trove of free muscle pics, BigMuscle.com has served 500,000,000 page views in the relatively long time that it's been a gay internet staple.
You don't have to create your own profile to browse the tens of thousands of existing ones, but if you want to join the site, doing so is completely free. BigMuscle (like its specialty spin-offs BigMuscleBears and BigMuscleLeather) is simply designed but powerfully addictive as one profile has a dozen or so shirtless or nude pics and leads to many other profiles as users add others to their lists of "friends" and "guys I like". An hour's browsing often uncovers the profile of a famous porn star, or even someone you may know, as you can search by locality.
It's very much a user-operated site, as there's no internal Java chatrooms or instant messaging service. Instead, users post their email or messenger addresses - protected by anti-spam technologies - and get in touch with each other off site.