*R for any alkyl group or even any organyl group (Alk can be used to unambiguously indicate an alkyl group)
*Me for the methyl group
*Et for the ethyl group
*Pr, n-Pr, or Pr for the (normal) propyl group (Pr is also the symbol for the element praseodymium.  However, since the propyl group is monovalent, while praseodymium is nearly always trivalent, ambiguity rarely, if ever, arises in practice.)
*i-Pr or Pr for the isopropyl group
*All for the allyl group (uncommon)
*Bu, n-Bu or Bu for the (normal) butyl group
*i-Bu or Bu (i often italicized) for the isobutyl group
*s-Bu or Bu for the secondary butyl group
*t-Bu or Bu for the tertiary butyl group
*Pn for the pentyl group (or Am for the synonymous amyl group, although Am is also the symbol for americium.)
*Np or Neo for the neopentyl group (Warning: Organometallic chemists often use Np for the related neophyl group, PhMeC–. Np is also the symbol for the element neptunium.)
*Cy or Chx for the cyclohexyl group
*Ad for the 1-adamantyl group
*Tr or Trt for the trityl group