Most of them are a bit irritating because of limitations on customising table and column widths. The last method offers the most flexibility and produces quite nice output.

The code for this is online at https://gist.github.com/benmarwick/8ad99f35d5e4caa06492

my_data <- head(iris)
names(my_data) <- c(letters[1:ncol(iris)])
library("knitr")
kable(my_data)
a b c d e
5.1 3.5 1.4 0.2 setosa
4.9 3.0 1.4 0.2 setosa
4.7 3.2 1.3 0.2 setosa
4.6 3.1 1.5 0.2 setosa
5.0 3.6 1.4 0.2 setosa
5.4 3.9 1.7 0.4 setosa 
library("xtable")
print(xtable(my_data), type = "html", include.rownames=FALSE, html.table.attributes=list("border='0' cellpadding='5' "))
a b c d e
5.10 3.50 1.40 0.20 setosa
4.90 3.00 1.40 0.20 setosa
4.70 3.20 1.30 0.20 setosa
4.60 3.10 1.50 0.20 setosa
5.00 3.60 1.40 0.20 setosa
5.40 3.90 1.70 0.40 setosa 
library(xtable)
print(xtable(my_data), type = 'html')
a b c d e
1 5.10 3.50 1.40 0.20 setosa
2 4.90 3.00 1.40 0.20 setosa
3 4.70 3.20 1.30 0.20 setosa
4 4.60 3.10 1.50 0.20 setosa
5 5.00 3.60 1.40 0.20 setosa
6 5.40 3.90 1.70 0.40 setosa 
library(xtable)
print(xtable(my_data), type = 'html', html.table.attributes = '')
a b c d e
1 5.10 3.50 1.40 0.20 setosa
2 4.90 3.00 1.40 0.20 setosa
3 4.70 3.20 1.30 0.20 setosa
4 4.60 3.10 1.50 0.20 setosa
5 5.00 3.60 1.40 0.20 setosa
6 5.40 3.90 1.70 0.40 setosa 
library("pander")
pandoc.table(my_data)
a b c d e
5.1 3.5 1.4 0.2 setosa
4.9 3 1.4 0.2 setosa
4.7 3.2 1.3 0.2 setosa
4.6 3.1 1.5 0.2 setosa
5 3.6 1.4 0.2 setosa
5.4 3.9 1.7 0.4 setosa 
library("pander")
pandoc.table(my_data, split.cells = 5)
a b c d e
5.1 3.5 1.4 0.2 setosa
4.9 3 1.4 0.2 setosa
4.7 3.2 1.3 0.2 setosa
4.6 3.1 1.5 0.2 setosa
5 3.6 1.4 0.2 setosa
5.4 3.9 1.7 0.4 setosa 
pander::panderOptions('table.split.table', 350)
pander::pandoc.table(my_data, style="rmarkdown")
a b c d e
5.1 3.5 1.4 0.2 setosa
4.9 3 1.4 0.2 setosa
4.7 3.2 1.3 0.2 setosa
4.6 3.1 1.5 0.2 setosa
5 3.6 1.4 0.2 setosa
5.4 3.9 1.7 0.4 setosa 
library("ascii")
## 
## Attaching package: 'ascii'
## 
## The following object is masked from 'package:pander':
## 
##     Pandoc
print(ascii(my_data), type = 'pandoc')
## Warning in rep(rownames, length = nrow(x)): 'x' is NULL so the result will
## be NULL
## Warning in rep(colnames, length = ncol(x)): 'x' is NULL so the result will
## be NULL
a b c d e
1 5.10 3.50 1.40 0.20 setosa
2 4.90 3.00 1.40 0.20 setosa
3 4.70 3.20 1.30 0.20 setosa
4 4.60 3.10 1.50 0.20 setosa
5 5.00 3.60 1.40 0.20 setosa
6 5.40 3.90 1.70 0.40 setosa 
library("htmlTable")
htmlTable(my_data, col.rgroup = c("none", "#F7F7F7"))
a b c d e
1 5.1 3.5 1.4 0.2 1
2 4.9 3 1.4 0.2 1
3 4.7 3.2 1.3 0.2 1
4 4.6 3.1 1.5 0.2 1
5 5 3.6 1.4 0.2 1
6 5.4 3.9 1.7 0.4 1 
library(hwriter)
hwrite(my_data, border=0)
[1] “
a b c d e
1 5.1 3.5 1.4 0.2 setosa
2 4.9 3.0 1.4 0.2 setosa
3 4.7 3.2 1.3 0.2 setosa
4 4.6 3.1 1.5 0.2 setosa
5 5.0 3.6 1.4 0.2 setosa
6 5.4 3.9 1.7 0.4 setosa

"

This one is the most useful, and has a nice guide to customisation here: http://www.ebi.ac.uk/~gpau/hwriter/

library(hwriter)
cat(hwrite(my_data, border = 0, center=TRUE, table.frame='void', width='300px', table.style='padding: 50px', row.names=FALSE, row.style=list('font-weight:bold')))
a b c d e
5.1 3.5 1.4 0.2 setosa
4.9 3.0 1.4 0.2 setosa
4.7 3.2 1.3 0.2 setosa
4.6 3.1 1.5 0.2 setosa
5.0 3.6 1.4 0.2 setosa
5.4 3.9 1.7 0.4 setosa