I've been working mostly with MS Visual Studio .NET tools and technologies, but started to wonder out of curiosity if there are alternatives to it, especially open source ones, since it seems like for every payed products there are free open source alternatives.
Yes, there are alternatives. Best known is the Mono.Net framework and MonoDevelop for the development environment. The other, arguably lesser known, is SharpDeveloper.
I've actually used both. At the time, MonoDevelop wasn't too mature, but that's meanwhile changed. It's actively developed, runs on a plethora of operating systems and is a fine IDE. The activity on the forums are good signs of the wide, enthusiastic and ever growing user base.
SharpDeveloper is a small but rather stable IDE. It is lightweight, loads quick and can be a relief to work with. However, for any larger projects, I noticed that I missed the enterprise features of Visual Studio. SharpDeveloper contains a graphical GUI for developing your WinForms applications.
Last time I looked, neither contains any of the wizards you may have grown accustomed to, nor the ASP.NET graphical design service. I never bothered to use either (other then for knowing why it is there and to learn why it is wise to ignore it), but if you love wizards and graphical aids, you may find these open source IDE-alternatives a bit Spartan.
That said, they're well worth a look and are stable and mature enough to be used in production.
Update: expanded, fixed some errors