You could store it in a separate one column, one item list  that is
used for just that purpose, using a SPD workflow that fires on new
records being added in the list that you are worried about keeping
track of. Just replace what the stored ID is in your 2nd little list
with the new one generated when a new list item is created in your
target list. It is cheating like crazy, but keeps your solution free
of custom coding. Then you can do a lookup on that list item to get
the ID for whatever you want it for.

There are several different methods to do this - basically you just need
to grab the highest valued ID column value. If you have more specifics
about what you are trying to do and what tools you have at your disposal
we can give a more detailed answer...

Once you have the rows in an SPListItemCollection, you can do a count on the rows and get the ID from the last one, putting it into a variable, like this (I use a string variable, for simplicity with casting):

string myCAMLquery = @"
<View>  <Query>     <OrderBy>        <FieldRef Name='Created' Ascending='False' />     </OrderBy>  </Query>  </View> ";
SPListItemCollection myListColl = myList.GetItems(myCAMLquery);
int count = myListColl.Count-1; // because it starts at 0
string myID = myListColl[count]["ID"].ToString();