Simple, asynchronous Slack log4net appender. Grab the NuGet package here.
<appender name="SlackAppender" type="Log4Slack.SlackAppender, Log4Slack">
<WebhookUrl value="https://{your company}.slack.com/services/hooks/incoming-webhook?token={your token}" /> <!-- Your Slack webhook URL -->
<Channel value="#testing" /> <!-- Your Slack channel to post to -->
<Username value="Log4Slack" /> <!-- The username to post as -->
<IconUrl value="{your icon url}" />
<IconEmoji value=":ghost:" /> <!-- Include an emoji to add a little character to your message -->
<AddAttachment value="true" /> <!-- Include an attachment on the Slack message with additional details -->
<AddExceptionTraceField value="true" /> <!-- If an exception occurs, add its trace as a field on the message attachment -->
<UsernameAppendLoggerName value="true"/> <!-- Append the current logger name to the Slack username -->
</appender>