Very Rarely does Bovada seem to offer a freeroll that is not restricted in some way.
The poker network making the most waves at the current time in terms of freerolls is the Winning Poker Network which includes the
poker sites America's CardRoom, Black Chip Poker, and True Poker. They have a unique freeroll structure where a new freeroll is started as soon as the current freeroll has at least 270 players and has gone through a few levels of late registration. In this way, we see new freerolls starting every half hour on the Winning Poker Network.
On top of that, BCP and ACR both have a bi-monthly competition which is completely free. The top 10 freeroll winners over a period of time (2-weeks) win tournament entries valued at over $100 while 1st place gets a $200+ tournament package. So the more freerolls you play and cash in the more freeroll "points" you earn which propel you to the top of hte competition list.
On top of the freerolls on demand that are offered by the winning poker network, every day at 1pm and 3pm there is a freeroll which can gain you entry to a
real money tournament of some sort. These freerolls usually allow rebuys and addons but always free to initially enter.