DevXpress XtraEditors for free
Through my employer I got introduced to the DevExpress component suite, which is by far the most comprehensive and comfortable component suite I’ve tried for Windows Forms. Unfortunately, it’s really expensive.
Turns out, however, that they’re giving away the core set of editors and controls for free. Of course, it doesn’t include their comprehensive datagrid control, but a lot of the controls included in the free package can improve the GUI a lot.
Get all 60 free controls at http://www.devexpress.com/Products/Free/WebRegistration60/
PS: DevXpress ships their controls with a lot of really garish skins. Set it to use the WindowsXP style and it’ll assume the default look on all Windows themes. (Classic, Luna, Aero, etc)