The title says it all. Check Log4Net site or MongoDB site if you need more info.
This is the official .NET implementation for the log4mongo project
Get it on NuGet, or download sources and run build.cmd to build
<appender name="MongoDBAppender" type="Log4Mongo.MongoDBAppender, Log4Mongo">
<!--
MongoDB database connection in the format:
mongodb://[username:password@]host1[:port1][,host2[:port2],...[,hostN[:portN]]][/[database][?options]]
See http://www.mongodb.org/display/DOCS/Connections for connectionstring options
If no database specified, default to "log4net"
-->
<connectionString value="mongodb://localhost" />
<!--
Name of the collection in database
Optional, Defaults to "logs"
-->
<collectionName value="logs" />
<field>
<name value="timestamp" />
<layout type="log4net.Layout.RawTimeStampLayout" />
</field>
<field>
<name value="level" />
<layout type="log4net.Layout.PatternLayout" value="%level" />
</field>
<field>
<name value="thread" />
<layout type="log4net.Layout.PatternLayout" value="%thread" />
</field>
<field>
<name value="logger" />
<layout type="log4net.Layout.PatternLayout" value="%logger" />
</field>
<field>
<name value="message" />
<layout type="log4net.Layout.PatternLayout" value="%message" />
</field>
<field>
<name value="mycustomproperty" />
<layout type="log4net.Layout.RawPropertyLayout">
<key value="mycustomproperty" />
</layout>
</field>
</appender>