I didn't actually find A Funny Thing Happened On The Way To The Forum that funny. It had its amusing moments, but it wasn't really laugh out loud material. It did bring back fond memories of Up Pompeii!, though.
For those who haven't seen it, Up Pompeii! (usual wikipedia caveats apply) was a BBC sitcom starring Frankie Howerd (who was Pseudolus in the London version of A Funny Thing Happened On The Way To The Forum) as Lurcio, slave to the Pompeii household of Ludicrus Sextus, Amonia, Nausius, and Erotica. Lurcio's attempts to bring the audience a bit of classical culture were always foiled by the fact that he never managed to finish the prologue to whatever story he wanted to tell because the complex machinations of various members of the household kept getting in the way.
To my 13-year-old self it was hilarious, and as I near 50, the clips on the BBC's page devoted to the programme (scroll down for the clips) are still pretty funny. What made it funny was, of course, Frankie Howerd's asides to the audience, which weren't so much asides as the whole point, and the situations forever teetering on the brink of disaster until he managed to get them back under control. Another favourite character, seen making a typical entrance at the end of the first clip, was Senna the Soothsayer. They should have made many more episodes, rather than the dire film version. Woe, woe and thrice woe, indeed.
DVDs of the series, together with other Frankie Howerd performances, are available from amazon.co.uk, but not it would seem from the American amazon.