What is the best way to escape HTML-specific characters in a string (PowerShell)?

There's a class that will do this in System.Web.

Add-Type -AssemblyName System.Web
[System.Web.HttpUtility]::HtmlEncode('something <somthing else>')

You can even go the other way:

[System.Web.HttpUtility]::HtmlDecode('something <something else>')

Starting with PowerShell 3.0, use [System.Net.WebUtility] for any of the four common operations:

[System.Net.WebUtility]::HtmlEncode('something <somthing else>')
[System.Net.WebUtility]::HtmlDecode('something &lt;somthing else&gt;')
[System.Net.WebUtility]::UrlEncode('something <somthing else>')
[System.Net.WebUtility]::UrlDecode('something+%3Csomthing+else%3E')

[System.Web.HttpUtility]::HtmlEncode is the common approach previous to .NET 4.0 (PowerShell 2.0 or earlier), but would require loading System.Web.dll:

Add-Type -AssemblyName System.Web

Starting with .NET 4.0 (PowerShell 3.0) [System.Web.HttpUtility]::HtmlEnocde internally calls [System.Net.WebUtility]::HtmlEncode, therefore it makes sense to leave out the middle man (System.Web.dll).