Rowan Atkinson. Turned up to collect his daughter from a big children's party I was nannying at, circa 2003.
Very unassuming and pleasant. We exchanged amused smiles because he knew I knew who he was, and I knew he knew I knew...but he was just there as a regular Dad and I respected his privacy.
Thom Yorke (Radiohead) Browsing art books in Blackwells, Oxford. Made my day, that did. He knocked two hardbacks off a table and swore loudly.
Tony Robinson. (in his Time Team era) Mooching around a picture-framing shop in our tiny town. The bloke behind counter went bright pink with delight, gesticulated wildly and stage whispered "Baldrick's ruddy here".
Johnny Walker (legendary dj/broadcaster). In a Soho pub. Surprisingly shy.
Maria Von Trapp (the real one, inspiration behind The Sound Of Music). In a Vermont restaurant. By then she was a very elderly lady, wrapped up in a huge coat, quietly sitting with very talkative friends.