I just started using Brewer's Friend. (I was previously using BeerSmith and BrewTarget.) I like that the software web-based, so I can reference it at homebrew club meetings or brew sessions at a friend's house. 

**Features:**

- Recipe calculator including expected OG and FG, IBUs, SRM color, and other common measures of a beer. (This includes multiple formulas/methods for calculating each.)
- Recipe scaling.
- Water conditioning calculator for areas with regionally variable water supplies.
- Shopping and brewday checklists.
- Batch logs for entering gravity, tasting notes, volume collected, etc.
- Tracker for progress of batch.
- Style guides.

**Disadvantages:**

- The recipe base is rather scant at this point.
- Unit conversions between imperial and metric (temp, volume, etc) are a little clunky.
- No mobile phone interface.

**Advantages:**

- Portable. Create a recipe at work, view it at home, brew it at a friend's house.
- Under active development. New features seem to come around frequently.

Disclaimer: I'm a web developer who found this homebrew site through some of the web development siblings on the Stack Exchange network. I appreciate that Brewer's Friend is a small start-up site under active development and that probably colors my view.