In the second episode of Gainax's Panty & Stocking with Garterbelt, we have spotted some cameos from the entire Gurren Lagann team, and some cameos coming from various characters. I think I will just let this picture do the talking.
Aside from that, on the said segment, Geek Boy seems like a direct reference to the Ghostbusters! Take note, this anime is not suitable for younger audiences due to its content... even though the artwork looks too innocent.