As mentioned in the entry on JAM, CoFesta is a blanket event that runs from September to the end of October. Included in this blanket is the Tokyo International Film Festival (TIFF) and CEATEC (both covered by bartman905), JAM, Tokyo Game Show and Entamatsuri.
Entamatsuri (Entertainment Festival) is a multi-part event scattered across Akihabara, with only parts of it being held in the UDX building. I tried going to the event space in the Sofmap (8th) building nearby, but the idols scheduled to appear on stage seemed to be on every other hour or so and there was nothing happening when I could get out during my breaks. What I did catch included a live song and dance routine in the plaza in front of Tully's on the ground floor of the UDX, a bit of the madhouse on the 2nd floor at UDX Square, and the manga magazine display in one of the exhibit rooms on the 4th floor (near the Tokyo Anime Center (TAC). The TAC itself wasn't doing anything interesting this time, instead focusing on promoting the Japanese dubbed version of the Pixar film "Up".
First, the madhouse. I didn't bother looking at the guest list this time, so I can't comment on who they were. The rest area was converted into a stage and theater-seating space to hold a few hundred people. Outside, the few hundred people at a time were lined up around the building, waiting to get in for each guest (different guests every hour or so). (They also had some anime goods and toys available for sale in the booths in the walkway.)
Second, the live show in front of Tully's. A small stage was set up, and when I arrived about 100 people or so were packed in front of the stage jamming to a soft pop singing group. Maybe 20 of the audience (all young guys) were doing a synchronized dance routine, indicating that they'd probably been following these girls around to other events for a while. Again, I didn't bother trying to find out who the act was - the music sounded too sugary and sappy for my tastes, and the performance was only mildly interesting.
There were a fair number of people between me and the stage and I had to hold the camera over my head and try shooting blindly several times to get even a remotely good shot. After several shots, some event staff member came up to tell me to stop. Kind of funny, because the staff at events like this don't bother worrying about people using cell phone cameras - they're only concerned if the camera looks too much like a camera. It's funny because a lot of cell phones now have better-quality cameras than the one I'm using. Anyway, the guy didn't force me to erase the pictures, and I was just turning the camera off anyway to continue walking on to Sofmap.