True examples of common variation due to Mendelian Inheritance
Earwax consistency is an example of a monogenic trait with dimorphic phenotypes in humans -- "wet" and "dry" -- where both phenotypes are globally common, though allele ratios are highly variable between human populations.  The population genetics and molecular basis of this trait are discussed at length by Dr. John H. McDonald as part of a series on the myths of human genetics (archived version here).  I reformat his discussion in my answer.



In The dimorphism in human normal cerumen, Matsunaga summarizes 30 years of research into the earwax of Japanese families, with two major observations:


Dry earwax parents never have wet earwax children, suggesting the genetics underlying the wet phenotype are dominant to those underlying the dry phenotype.
Because the allele for wet earwax is relatively rare in the sampled families, it is likely that most wet earwax parents are heterozygotes.  Following from this, if the trait is monogenic with two allele variants, we expect a 3:1 ratio of wet to dry children in wet × wet parent crosses, and a 1:1 ratio of wet to dry children wet × dry parent crosses.  Indeed, this is observed. 


$$\begin{array}{c|c|c|} 
\text{parents} &amp; \text{wet children} &amp; \text{dry children} \\ \hline
\text{w×w} &amp; 35 &amp; 12 \\ \hline
\text{w×d} &amp; 205 &amp; 195 \\ \hline
\text{d×d} &amp; 0 &amp; 634 \\ \hline
\end{array}$$

These results were later corroborated in Native Americans --
Cerumen in American Indians: Genetic Implications of Sticky and Dry Types

$$\begin{array}{c|c|c|} 
\text{parents} &amp; \text{wet children} &amp; \text{dry children} \\ \hline
\text{w×w} &amp; 32 &amp; 6 \\ \hline
\text{w×d} &amp; 20 &amp; 9 \\ \hline
\text{d×d} &amp; 0 &amp; 42 \\ \hline
\end{array}$$

And in a different Japanese cohort --
Distribution and inheritance of earwax types: a study on inhabitants in Awa District, Chiba Prefecture

$$\begin{array}{c|c|c|} 
\text{parents} &amp; \text{wet children} &amp; \text{dry children} \\ \hline
\text{w×w} &amp; 27 &amp; 3 \\ \hline
\text{w×d} &amp; 137 &amp; 109 \\ \hline
\text{d×d} &amp; 0 &amp; 345 \\ \hline
\end{array}$$

In the early 2000s, the molecular basis of human earwax consistency was elucidated in three major publications.  


Toshida et al. mapped a single locus on chromosome 16
Yoshiura et al. identified a SNP at position 538 in the coding region of ABCC11.  At this site, G corresponds to a wet earwax phenotype, whereas A leads to a dry earwax phenotype.  A less common allele representing a deletion of 27 nucleotides in exon 29 also yields a dry phenotype.  
Toyoda et al. found that the 538G>A transition encodes arginine in place of glycine, resulting in a loss of glycosylation in the mutant protein, leading to deficient cellular localization, rapid degradation, and loss of function.  




To directly address your question,


  Are there any examples of human phenotypes that are due to mendelian inheritance where all of the phenotypes are common?


I point you to the tables above, as well as Table 1 from Cerumen Phenotypes in Certain 
Populations of Eurasia and Africa, which shows that the frequency of the dry allele ranges from 59% to 89% across diverse human populations --  



To conclude, human earwax consistency is a dimorphic trait controlled by a single gene, where both phenotypes are common across human populations.