Judy Greer - Denise
Always a guest star, never a serious regular, Judy Greer has been in a lot of tv shows and movies. She appeared as Phil's ex-girlfirend, Denise, in season 1, where despite his marriage to Claire, she proceeded to throw herself at him for the entirety of the episode.
Rob Riggle - Gil Thorpe
Rob plays a real estate agent and Phil's nemesis. He's appeared sporadically over the years but was first introduced in season 4. He's not above stealing clients or business opportunities, and he'll brag about it and insult you as he goes.
Leslie Mann - Katie
Leslie Mann is amazing in everything she does and Modern Family was no exception. She played Katie, a woman Cam bet he could pick up in a bar in season 3. They both ended the night happy as Cam won the bet and Katie found the gay friend she's always wanted.
Kelsey Grammer - Ringmaster Keifth
Of course, Cam's ex-boyfriend would be a ringmaster named Keifth. In season 8, Mr. Grammer portrayed a dedicated circus performer. The exchanges between him and Cam were hilariously wistful as they reminisced about their time spent under the big tent.
Dot Marie Jones - Louise
Also in season 8, Dot had the difficult job of telling Cam Fizbo was on the loose. Knowing his beloved character was being tarnished, Cam couldn't focus on the bowling championship that Dot's character, Louise, was also participating in. After some stellar detective work by Cam and Mitchell, we learned Louise was the one donning the costume and terrorizing people.
Nathan Lane - Pepper Saltzman
Another amazing character, with an even more amazing name, is Pepper Saltzman played by Nathan Lane. We met him in season 2 when Cam and Mitchell were trying to get out of one of his brunches. He appears sporadically and is known for his lavish theme parties.