C# Signature:

[DllImport("user32.dll")]
static extern short GetKeyState(int nVirtKey);

VB Signature:

<DllImport("user32.dll")> _
Public Function GetKeyState(ByVal nVirtKey As Integer) As Integer
End Function

User-Defined Types:

None.

Notes:

Here are some sample values for nVirtKey:

const int VK_SHIFT = 0x10;   // Use Keys.ShiftKey
const int VK_CONTROL = 0x11; // Use Keys.ControlKey
const int VK_MENU = 0x12;    // Use Keys.Menu

Tips & Tricks:

'Use the System.Windows.Forms.Keys enumeration instead of VK codes,
'but make sure you use the right one! (Keys.ShiftKey not Keys.Shift, etc.)

Sample VB Code:

Imports System.Windows.Forms

Public Function IsKeyDown(ByVal keys As Keys) As Boolean
    Return (GetKeyState(CInt(keys)) And &H8000) = &H8000
End Function

Public Function IsKeyOn(ByVal keys As Keys) As Boolean
     Return (GetKeyState(CInt(keys)) And &H1) = &H1
End Function

Sample C# Code:

using System.Windows.Forms;

public static bool IsKeyLocked(Keys keyVal) {
     switch( keyVal ) {
    case Keys.NumLock :
    case Keys.Capital :
    case Keys.Scroll :
    case Keys.Insert :
        return (GetKeyState((int)keyVal) & 0x8001) != 0;
     }
     return false;
}

public static bool IsKeyPressed(Keys keyVal) {
     return GetKeyState((int)keyVal) < 0;
}

// --- Added by Kasper ---

    /// <summary>
    /// This method tests whether a key is down or not.
    /// </summary>
    /// <param name="k">The key to test.</param>
    /// <returns>True if the keys is down.</returns>
    public static bool IsKeyDown(System.Windows.Forms.Keys k)
    {
        short state = GetKeyState((int)k);
        if((state & 0x8000) == 0x8000) //It's down
            return true;
        return false;
    }
    /// <summary>
    /// This method tests whether a key is on or not.
    /// </summary>
    /// <param name="k">The key to test.</param>
    /// <returns>True if the keys is on.</returns>
    public static bool IsKeyOn(System.Windows.Forms.Keys k)
    {
        short state = GetKeyState((int)k);
        if((state & 0x0001) == 0x0001) //It's on
            return true;
        return false;
    }

Alternative Managed API:

Do you know one? Please contribute it!