Freeroll Tournaments are free to enter but provide real prizes. And if they sound good, they are. There's no real problem apart from the that they're immensely popular, and are usually packed with hundreds and sometimes thousands of players all clamouring for the prizes.
Generally, the bigger the prize, the more people you'll get playing. The Paradise Poker Million Dollar Freeroll is the ultimate example. As the name suggests, it's a free-to-enter tournament that provides an amazing $1 million first prize. The downside? You'll have to battle past thousands of players to qualify for the final. And then past thousands more to get to the final table (usually played live in an exotic location).
The catch? There isn't one. You're not paying any money, and even if you don't win you'll get invaluable tournament experience. However, the nature of the freeroll means that the play is even looser than the loose internet play. Which means it's incredibly loose - especially in the first hour of the tournament when some players decide the best strategy is to double through quickly and often. Try to avoid any confrontation early on, unless you're in possession of the nuts.