1 A few masculine nouns (listed here)
2 Some neuter nouns (listed here)
3 Masculine and neuter nouns ending in -tum, e.g. der Reichtum, das Altertum.

These are the masculine nouns that don’t end in -tum:

der Geist /der Mann /der Strauch
der Gott /der Mund /der Wald
der Leib /der Rand /der Wurm

This is a sample of the neuter nouns but there are more

das Amt /das Feld /das Haus /das Loch
das Bad /das Gehalt /das Huhn /das Maul
das Band /das Geld /das Kalb /das Nest
das Bild /das Gesicht /das Kind /das Rad
das Buch /das Gespenst /das Lamm /das Schloss
das Dach /das Glas /das Land /das Tal
das Dorf /das Glied /das Licht /das Volk
das Ei /das Grab /das Lied /das Wort*

* Also found in Group 2 but with a different meaning, i.e. Wörter refers to indiviual 
words, not words as a collective, e.g. das Wörterbuch (dictionary).