Most of them want to hijack your home page and search engine and sometimes install toolbars. They often want to install other software. In that sense, none of these sites is any different than any other site that downloads software for the PC. Even utilities want other applications downloaded so ALL must be viewed with caution.
The key is to pay attention and READ each page to be sure what you're authorizing or not authorizing. Jenkat is probably the most confusing since you have to accept to even get TO the game, much less the garbage that is offered as well (included or separate).
I follow the "Freebie Circuit", as I call it, of 5 sites - Jenkat, MyPlayCity, FreeGamePick, MyRealGames and GameTop.
There are some name differences among all of them but, it is true that GameTop is the most common. Some differences are among all of them but separate from the "original" developer. For example, all of the circuit refer to "Snowy Bear" from Alawar as "Milky Bear". I'm guessing that is at Alawar's insistence. Also, Alawar's change of "Magic Ball" to "Smash Frenzy" is another confusion factor.
I have a spreadsheet that has all the games and in cases where different names are involved, both names are given and a note of "different names" is included.
In one way, MyPlayCity (usually referred to as MPC) is the most messy install and has been for years. There are normally anywhere from 12 to 15 files and 1 folder that are total garbage and can be removed without affecting your game in any way. Some are new such as icons for other games! Most of the sites have 2 or 3 garbage files included but none like MPC.
As with anything else, watch what you do and be willing to change because the install can and will change over time. I almost dropped FreeGamePick because it appeared to try to force you to install other software. A contact with them resulted in instructions on how to avoid the problem so they stayed on my list of freebie sites. They have recently added 29 games to their list.
Just for info -- based on my current count (not guaranteed to be accurate), the 5 sites each have the following number of free games:
Jenkat - 478 (only showing 475 with 3 obviously having issues with the new installer)
MyPlayCity - 1084
FreeGamePick - 238
MyRealGames - 271
GameTop - 318
No - I don't have all by any means. The total number of unique games among them is 1569.