Are the any open source or free software similar to messagingtoolkit ?
If you are looking for open source solution, you can try out smslib from http://code.google.com/p/smslib/. It is written in Java but there is a .NET port using IKVM.NET.
Also, my previous library ATSMS, which is written in VB.NET is also still available at http://www.twit88.com/home for download, but there is not support provided.
Gnokii (http://www.gnokii.org/) is another open source solution written in C which is initially for Nokia phones.
There is also a free but not open source library from http://www.scampers.org/steve/sms/ which is also written in C#.
You can also use Microsoft SMS Sender (http://www.microsoft.com/globaldev/outreach/dnloads/smsSender.mspx) to send SMS.
There are other free/open source solutions which I have not tested yet
- Gammu at http://www.gammu.org/
- gsmlib at http://www.pxh.de/fs/gsmlib/
- Kannel at http://www.kannel.org/
- OpenSMPP at http://sourceforge.net/projects/smstools/
None of the open source solutions support MMS. For messagingtoolkit, MMS sending will be supported in later version.