1. Chronoforms
The definite choice besides BreezingForms is this one fine non-commercial extension. A bit "out-of-date" but due to its concepts one of the most popular extensions for Joomla!®. It's quite simple: you create your own form HTML, copy and paste it into the form manager, the rest is done by Chronoforms. Awesome.
2. Proforms
Proforms is a relative young commercial form extension for Joomla!® and targets the non-experienced users. Its advantage is the ease-of-use but also provides an app system to get enhanced. The apps are provided by the developers themselves and are mostly commercial. Unlike BreezingForms, support is limited on domains, keep that in mind if you need it for plenty of installations across different domains.
3. RSform pro
This commercial extension is one of the oldest available ones for Joomla!®. Though technically not cutting-edge, it is meant for the not-so-experienced user and has basic features but still does its job pretty well. Similar to Proforms, support is given on limited domains, but there are plans to have support for unlimited domains, too.
Blue Flame Forms
Another fellow form developer offers this very feature-rich commercial extension. There is plenty of stuff to discover. Personally, I like the wizard form creation interface and focus on things like different anti-spam systems.
forms-for-joomla.com
Update:
4. JxiForms
Unfortunately, Blue Flame Forms is discontinued, instead we suggest JxiForms from ReadyBytes. It's a pretty easy to use and simple featured form builder, very good for basic tasks.
5. Fabrik
The absolute forms expert tool and a world of its own. This free extension is a hybrid between form creation and a content construction kit. If you need to create complex form based applications, then besides BreezingForms this is another alternative worth trying.
There are plenty of other form extensions for Joomla!® available but the collection above are definitely the best ones available at this time.