1. Lempel-Ziv compression algorithms (both LZ77 and LZ78). They are practical, even if one code their basic versions will get good compression ratio. Moreover, they opened the whole big chapter of data compression (LZW, LZSS and more). 2. The algorithm behind rsync. Nice one. 3. Jarvis algorithm for finding convex hull. 4. Speaking of data structures I like xor-linked list, it's neat.

[1] https://en.wikipedia.org/wiki/LZ77_and_LZ78 [2] https://en.wikipedia.org/wiki/Rsync#Algorithm [3] https://en.wikipedia.org/wiki/Gift_wrapping_algorithm [4] https://en.wikipedia.org/wiki/XOR_linked_list