Micro bar - Austurstræti 6, 101 Reykjavik, Iceland
Strange bar, since it looks like a small hotel bar at first. The looks deceive you, since they have the best beer selection in all of Reykjavik. Usually 8 Icelandic micro brews on tap + 1 hand pump. They also have an excellent bottle selection, around 100+ bottles, decent sour selection (around 10 different ones available).
The only downside is the price, a pint of their regular beer is around $9, a pint of Mikkeler Simcoe IIPA from a hand pump was $18, 3 Founteinen Oude Geuze 375 ml was $18 as well.

Kaldi bar, Laugarvegur 20b, 101 Reykjavik, Iceland
Bar operated by the Kaldi Microbrewery. They specialize in Czech style pilsners, and have 2 on tap, filtered and unfiltered. They also have their dark lager on tap, filtered and unfiltered, along with 3 other taps from microbrewery Gæðingur. Suffers from swarms of lumberjacks and flocks of hipsters with Kim Jong-Un haircuts.
500 ml of any beer is $10, 750 ml and 1l mugs available.

Hlemmur Square Bar, Hlemmur, 101 Reykjavik, Iceland http://www.hlemmursquare.com/the-bar-2/
Another hotel bar, crappy draft selection (only one decent beer), but good bottle selection with various beers from Mikkeler, Brew dog, and Rogue among others.