As mentioned in some of the answers, you can directly launch sln-files from the command line, i.e.
MySolution\MySolution.sln
The fastest way to start a solution is to take advantage of the tab-completion. Note that this only works if you have only one solution-file in the folder.
If you are in the solution folder:
*sln
tabenter
If you are not in the solution folder:
MySolution\*sln
tabenter
Or depending on your location you can tab-complete the solution folder as well:
MySol
tab\*sln
tabenter
This works in PowerShell as well with the following differences:
- the backslash is automatically inserted upon doing tab-completion on a folder
- you can use backslash or slash to mark a folder path