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.