A lot (meaning more than a few) is always two words. The phrase is often misspelled as alot. Allot is a verb that means to distribute by lot or shares.
Accept means to receive, or agree with; Except means to omit or to exempt. The two are commonly misused and often interchanged at little cost to standard usage.
Affect & effect are two highly misused and interchanged terms. As a verb affect means to influence. Effect as a verb means to cause but as a noun means result.
All right is always two words. Alright is not a word, although like altogether and already it looks like is should be and spellchecks are strangely allowing it.
A crepuscular animal is mostly active during twilight, which is at both dusk and dawn. House cats, bats, bears & ocelots are an examples of crepuscular animals.
It's and Its are often misused and confused in written English. It's means it is while its is the possessive pronoun meaning something belongs to something.
Onomatopoeia are words that imitate the sound they are describing and represent. Just a few examples could be: zap, buzz, beep, whoosh, bang, thump, thud, etc.
An oxymoron is a self contradictory statement, where the second portion of the statement negates the first. Common examples include a tall short person.
Sentence diagraming is the analysis of a sentence. Some authorities feel that it has little value, but its graphic form has a pleasing logistical balance.
Sesquicentennial is a word used to describe the passage of 150 years. It is often used in reference to an anniversary of an institution or a historical event.