Extension Methods for Trimming List
if (list.Count &lt;= maxCount)
{
    throw new InvalidOperationException("Items in list &lt;= maxCount, unable to trim list.");
}


This doesn't seem like an exceptional case to me. If the idea is to trim the list to a certain size and the list is already smaller than that size -- the job is already done! In that case I would replace this with return true;.



while (list.Count &gt; maxCount)
{
    T last = list.Last&lt;T&gt;();
    list.Remove(last);
}


This is the main guts of your method. Since the preceding if statement has already established that list.Count &gt; maxCount, you know that the loop is always going to run at least one. Therefore you could replace this with a do-while loop. This is a negligible performance improvement.

do
{
    T last = list.Last&lt;T&gt;();
    list.Remove(last);
}
while (list.Count &gt; maxCount)




catch
{
    return false;
}


Two problems here.


YAGNI
Using boolean values to indicate whether or not the method was successful is a design flaw. This harks back to the old HRESULT days which were universally loathed. Instead, let the exception do the job of alerting the calling code to errors (this is what exceptions are for). In the case that there is no exception, there is no use for the returned true value.




You could also consider returning a reference to the list itself from the method, allowing people to use method chaining syntax, which is all the rage these days - especially with LINQ..

public static List&lt;T&gt; TrimFirst&lt;T&gt;(this List&lt;T&gt; list, int maxCount)
{
    if (list.Count &lt;= maxCount)
        return list;

    do
    {
        T first = list.First&lt;T&gt;();
        list.Remove(first);
    }
    while (list.Count &gt; maxCount);

    return list;
}




One final thing. It's usually a good idea to defend against NullReferenceExceptions by asserting that your argument is not null.

if (list == null)
    throw new ArgumentNullException("list");


ArgumentNullExceptions are always preferable to NullReferenceExceptions.