Interoperating: Microsoft Pocket PC

Known to work:

Pocket PC 2003 (Premium Edition) and Windows Mobile 5.0 ship with a built-in L2TP/IPsec client (the VPN clients mentioned below support IPsec without L2TP). For details, see Jacco de Leeuw's article, Using Openswan with Pocket PC 2003 and Windows Mobile 5.0. This built-in L2TP/IPsec client supports both preshared keys and certificates. According to Microsoft, Pocket PC only supports certificates that are obtained through 'web enrollment' (which would dictate the use of a Windows Certificate Server). But with a utility called P12imprt you can also import a PKCS#12 file from any CA (and not just Microsoft's) to the Pocket PC.

Status unknown:

AnthaVPN (continuation of Movian) by Worldnet21 / Anthasoft
Apani Networks Windows 2003 PDA VPN Client for the Nortel VPN Router (formerly Nortel Contivity). Might only work with Nortel.
Certicom MovianVPN (discontinued since Oct 31, 2004).
Check Point VPN-1 Secure Client. Might only work with Check Point.
Funk AdmitOne (discontinued since Jan 1, 2004)
NCP Secure CE client _should_ work because it works with Astaro which uses Openswan. (Please report here if it works indeed).
Nortel Contivity VPN Client (seems to be an Apani OEM client)
SafeNet SoftRemotePDA (for Pocket PC 2002).